Quarterly Newsletter

2013 - IVG Newsletter First Quarter

The IVG Hospitals Quarterly Veterinary Newsletter features articles of interest to the veterinary medical community, written by veterinarians and veterinary specialists at our four locations.

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If You Haven't Considered Evaporative KCS… Let's Review a Few Tear Film Concepts Clara Williams, DVM, MS, DACVO KERATOCONJUNCTIVITIS SICCA IS AN UMBRELLA diagnosis that includes multiple conditions in which the corneal protection is not adequate. This inadequacy may be caused by abnormalities in the tear film (quantity and quality), or by changes in the blinking patterns. Age, inflammatory conditions and innervation abnormalities are among the triggering causes. The end result is a desiccated cornea prone to ulceration. To understand the several Kerato-Conjunctivitis sicca presentations, we must have a clear understanding of how the cornea maintains its integrity. Remember this diagram? Corneal Integrity Adequate Corneal Protection Normal Epithelial Loss and Normal Regeneration Last year, we talked about the cornea's dependence on adequate protection and normal exfoliation/regeneration dynamics to maintain its integrity. The two mechanisms for protection of the corneal surface include an adequate tear film and the action of blinking. The cornea is a fragile and demanding customer! One can sympathize with it because the cornea supports intraocular structures, refracts incoming beams of light, thanks to its curvature, and transmits rays of light, thanks to its transparency. The only way the cornea can maintain these characteristics is with the protection of the eyelids and the tear film. THE TEAR FILM If one is given the task of designing a nutritive fluid that must be transparent, must adhere, without evaporating too quickly to a mostly perpendicular hydrophobic, concave surface, one faces a great |5| Dr. Clara Williams practices at Massachusetts Veterinary Referral Hospital in Woburn, MA. challenge! Did I mention this fluid must also remove foreign material from the cornea and conjunctiva? The demands of adherence, nutrition and slow evaporation can be addressed with an equal number of layers in the tear film: the mucinous, hydrophilic and lipid layers. (See Fig. 1). • The mucin layer (0.02 to 0.05 micron) is the deepest layer of the tear film. It is produced by the apocrine conjunctival goblet cells and the deep gland (Harderian) of the nictitans. The mucin layer contains glycoproteins that lower the surface tension of the film permitting it to spread upon blinking. The mucin makes the hydrophobic surface of the cornea more hydrophilic. The mucin of the pre-corneal tear film adheres firmly to the anterior epithelium projections (microplicae and microvillae).The retention of the tear film on the cornea depends on the mucin layer, as well as on the integrity of the outermost layer of the corneal epithelium. • The hydrophilic (aqueous) layer (7microm) is the thickest layer (>60%). It is composed of approximately 98% water and 2% solids (inorganic salts, glucose, urea, proteins, glycoproteins and biopolymers). This layer performs the primary function of the tear film and is produced by the lacrimal glands (orbital and third eyelid) and the Harderian gland. The lacrimal nerve (branch of the trigeminal) with sympathetic and parasympathetic fibers innervates the lacrimal glands.

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